HfO2 Films Research Articles

Hafnium dioxide obtained by atomic layer deposition possesses the ability to induce the formation of biological apatite

The research explored the spontaneous formation of the BAp precursor on surfaces of HfO2 films obtained by Atomic Layer Deposition (ALD). It has been found that a careful selection of ALD growth conditions, followed by rapid thermal annealing (RTP) is crucial to achieve bioactivity of the films. Simulated Body Fluid experiment was used as a reliable test of the film's functionality as bone implant coating. SEM, XRD and XPS investigations proved that amorphous calcium orthophosphate structures were formed on the HfO2 films. These structures are of importance for biomineralization of bones. Physical and chemical characterization of the films was an integral part of the research. Such approach allowed us to achieve biomaterial functionality while maintaining the films’ quality. The finding has significance for future progress in personalized traumatological medicine.

Effect of Annealing Conditions on Resistive Switching in Hafnium Oxide-based MIM Devices for Low-power RRAM

Abstract This work presents resistive switching (RS) behaviour in HfO2-based low-power resistive random-access memory (RRAM) devices. A metal-insulator-metal (MIM) structure (Au/HfO2/Pt) was fabricated by sandwiching a thin insulating layer of HfO2 between Pt and Au electrodes. HfO2 films deposited by RF sputtering at room temperature were rapid thermally annealed in N2 ambient at 400 °C and 500 °C. Grazing angle X-ray diffraction (GIXRD), Field emission gun-scanning electron microscopy (FEG-SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were employed to analyse the phase, crystal structure, morphology, surface roughness and chemical composition of the HfO2 films. The bipolar RS could be observed in both as-deposited and annealed HfO2 film-based devices from I-V characteristics measured using a source meter. We have investigated the effect of annealing temperature and annealing ambient on the phase formation of HfO2 as well as the RS characteristics and compared with as-deposited film-based device. Annealed HfO2 film-based devices exhibited improved electrical characteristics, including stable and repeatable RS at significantly lower switching voltages (˂1 V) which indicates low power consumption in these devices. The relatively lower processing temperature of the HfO2 films and that too in the films deposited by physical vapor deposition (PVD) technique-RF magnetron sputtering makes this study significantly useful for resistive switching based non-volatile memories.

Time-Dependent Resistance of Sol–Gel HfO2 Films to In Situ High-Temperature Laser Damage

Laser damage in films under long-term high-temperature conditions is a significant concern for advancing laser applications. This study focused on HfO2 films prepared using the sol–gel method with HfCl4 as a precursor. It examined the effects of temperature on various properties of the films, including their optical properties, microstructure, surface morphology, absorption, and laser-induced damage threshold (LIDT). The prepared film demonstrated desirable characteristics at the high temperature of 423 K, such as high transmittance, low absorption, and high LIDT. As the duration of its high-temperature exposure increased, the LIDT of the films gradually decreased. An intriguing finding was that the film’s LIDT exhibited an exponential decay pattern with prolonged heating time. This observation could be attributed to the power-law increase in defects on both the internal and surface areas of the film as the duration of high-temperature exposure lengthened. Moreover, even after a 15-day heating period at 423 K, the film maintained an LIDT of 12.9 J/cm2, indicating its potential applicability in practical high-temperature environments. This study provided a general pattern and a universal formula for understanding the laser damage of sol–gel films at high temperatures over time. Furthermore, it opened possibilities for future developments of laser films suitable for extreme environments.

Effect of annealing on ion-beam-sputtered hafnium oxide thin films properties

HfO2 films are high laser damage-threshold and low-absorption thin-films that can be utilized for a broad range of optoelectronic and optical applications. In this study, HfO2 thin films were prepared via dual ion beam sputtering followed by annealing in the atmosphere at temperatures ranging from 200 to 700 °C. The optical properties, microstructure, film stress, and absorption loss of the HfO2 films at various annealing temperatures were then systematically characterized. The results revealed that the properties of the films were improved after annealing at temperatures below 500 °C. Specifically, at an optimal annealing temperature of 400 °C, the residual stress of the films was minimized close to zero and the absorption loss was as low as 1 ppm. However, at annealing temperatures reaching 600 °C, the films began to crystallize, leading to the deterioration of their optical properties. Optimizing the process parameters for HfO2 thin film deposition is crucial for developing the next generation of high-performance laser optics.

Role of ex-situ HfO2 passivation to improve device performance and suppress X-ray-induced degradation characteristics of in-situ Si3N4/AlN/GaN MIS-HEMTs

The role of ex-situ HfO2 passivation in in-situ Si3N4/AlN/GaN-based metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) to improve the device performance and protect the device from severe degradation under high-energy X-ray irradiation has been investigated. Here, the root cause and mechanism for the degradation have been studied utilizing different material/device characterizations. X-ray photoelectron spectroscopy results suggest that several Si-N bonds in stoichiometric Si3N4 and Hf-O bonds in HfO2 passivation film can be broken under X-ray irradiation, where numerous charges might be trapped easily and deteriorate the interface quality. Furthermore, due to the generation of chemical vacancies under X-ray, a band tail near the edge of valence band maximum in Si3N4 and HfO2 films can be created. The obtained results suggest that MIS-HEMT fabricated with an atomic layer deposited (ALD)-HfO2 (MIS-HEMT B) has relatively greater performance compared to MIS-HEMT fabricated without ex-situ HfO2 (MIS-HEMT A) according to the higher maximum drain current (Idmax), peak transconductance (gmmax) and lower current collapse, denoting the effectiveness of ALD-HfO2 to passivate the surface traps. It is noticed that X-ray irradiation induces numerous negative charges in the dielectric layers and degrades the device performance. Interestingly, after X-ray irradiation, the degradation rates in Idmax, gmmax, and current collapse are significantly lower in MIS-HEMT B compared to MIS-HEMT A, suggesting the potential irradiation hardening mechanism of ALD-HfO2 to protect the MIS-HEMTs from unexpected degradations during X-ray irradiation. Finally, these results emphasize the importance of high-quality dielectric films in MIS-HEMTs to enhance the device performance and safeguard the device under high-energy irradiation, especially in the space environment.

Enhancement of dielectric constant in Sm:Zr co-doped HfO2 films synthesized by cost-effective method

Sm, Zr co-doped HfO2 films were deposited on a p-type silicon wafer using a cost-effective spin coating method. The influence of Sm, Zr co-doping concentration variation on structural, compositional, morphological, and electrical properties was examined. The dominance of the orthorhombic phase (o-phase) in comparison to the monoclinic phase was noted to rise with the simultaneous increment of Sm doping concentration and decrement of Zr concentration. A decrease in oxygen vacancies was noted for all the Sm, Zr co-doped HfO2 films from XPS results. Nonporous and uniform distribution of grains was observed from Field Emission Scanning Electron Microscope (FESEM) and Atomic Force Microscopy (AFM) studies. Electrical studies of MOS capacitors based on Sm, Zr co-doped HfO2 films revealed a low leakage current. C-V studies exhibited an increment of dielectric constant and a decrement of interface trap densities. Further, to obtain insights into the microscopic features of Sm, Zr co-doped films, impedance and modulus studies have been performed. The obtained results imply the potential of tuning Sm, Zr co-doping concentrations in HfO2 towards stabilization of the orthorhombic phase and to enhance the dielectric constant of the films.

Crystalline phase control of ferroelectric HfO2 thin film via heterogeneous co-doping

Our study investigates heterogeneous co-doped HfO2 thin films integrated into metal-ferroelectric-metal stacks, achieved by incorporating multiple layers doped with various species during the atomic layer deposition process. This approach creates an artificial crystallization temperature gradient across the HfO2 film, influencing the preferred nucleation sites of HfO2 during rapid thermal processing. Our findings demonstrate that the phase composition of the annealed HfO2 film is primarily determined by heterogeneous or homogeneous crystallization processes. In cases of heterogeneous crystallization, where crystallization initiates from nuclei formed at electrode/HfO2 interfaces, grains predominantly crystallize in the orthorhombic phase. Conversely, grains are more likely to crystallize in the monoclinic phase if they originate from nuclei formed at the center of the HfO2 film. Additionally, we observe correlations between the texture of the HfO2 film and the texture of the electrodes.

Atomic-Scale Scanning of Domain Network in the Ferroelectric HfO2 Thin Film.

Ferroelectric HfO2-based thin films have attracted much interest in the utilization of ferroelectricity at the nanoscale for next-generation electronic devices. However, the structural origin and stabilization mechanism of the ferroelectric phase are not understood because the film is typically nanocrystalline with active yet stochastic ferroelectric domains. Here, electron microscopy is used to map the in-plane domain network structures of epitaxially grown ferroelectric Y:HfO2 films in atomic resolution. The ferroelectricity is confirmed in free-standing Y:HfO2 films, allowing for investigating the structural origin for their ferroelectricity by 4D-STEM, high-resolution STEM, and iDPC-STEM. At the grain boundaries of <111>-oriented Pca21 orthorhombic grains, a high-symmetry mixed-(R3m, Pnm21) phase is induced, exhibiting enhanced polarization due to in-plane compressive strain. Nanoscale Pca21 orthorhombic grains and their grain boundaries with mixed-(R3m, Pnm21) phases of higher symmetry cooperatively determine the ferroelectricity of the Y:HfO2 film. It is also found that such ferroelectric domain networks emerge when the film thickness is beyond a finite value. Furthermore, in-plane mapping of oxygen positions overlaid on ferroelectric domains discloses that polarization is suppressed at vertical domain walls, while it is active when domains are aligned horizontally with subangstrom domain walls. In addition, randomly distributed 180° charged domain walls are confined by spacer layers.

The piezoelectric activity in the HfO2 nanoclusters

In this paper the piezoelectric effects in infinite free-standing HfO2 film and HfO2 cluster under mechanical action are studied by methods of the density functional theory and first-principles pseudopotential based on own program code. The spatial distribution of valence electron density, the forces that act on an individual atom from the electronic and ionic subsystems, and Coulomb potential along transverse direction are calculated. It was found that a polarization charge distribution of valence electrons, which leads to piezoelectric effects, was observed in HfO2 nanofilms, while in HfO2 nanoclusters the charge distribution remained uniform both in the uncompressed and in the compressed state.

Low-thermal-budget crystallization of ferroelectric Al:HfO2 films by millisecond flash lamp annealing

We report the use of a low-thermal-budget annealing technique; flash lamp annealing (FLA), which provides an extremely short annealing time in the millisecond range, on the ferroelectric properties of Al-doped HfO2 (HAO) films. HAO annealed at 1000 °C with 5 ms shows a higher remanent polarization value of 24.9 μC cm−2 compared to rapid thermal annealing (RTA), without degradation of endurance. GIXRD shows a stronger peak intensity originating from the orthorhombic (o-) phase and is observed when using FLA, indicating the formation of a larger amount of the o-phase. We believe that this is a consequence of the low thermal budget of FLA, and that specifically FLA can minimize the relaxation of the compressive stress in the TiN electrodes, inducing a high tensile stress to the HAO films and therefore an enhancement of o-phase formation. These results indicate that FLA is a promising annealing method for HAO crystallization due to the enhancement of o-phase formation.

Ta5+-substitution effects on crystal structure and ferroelectric property in HfO2-based films

To investigate the Ta5+-substitution effects on crystal structure and ferroelectric property in HfO2-based films, Ta x Hf1−x O2+δ films with various film thicknesses and Ta content were prepared. The ferroelectric orthorhombic phase was formed in a wide film thickness range of 20–100 nm while in a narrow composition range of x = 0.10–0.14. These thickness-insensitive and composition-sensitive characteristics of Ta5+-substituted HfO2 film are similar to Y3+ rather than Zr4+. The X-ray photoelectron spectroscopy measurement suggests that the ionic state of Ta is not reduced and Ta x Hf1−x O2+δ film has an excess oxygen state. The excess oxygen may consist of a combination of oxygen vacancies and more interstitial oxygens. These defects facilitate the formation of the ferroelectric phasebut also decrease the breakdown voltage and increase the leakage current in Ta5+-substituted HfO2 films. On the other hand, the generation of excess oxygen indicates the possibility of controlling oxygen vacancies which deteriorate fatigue and retention properties.

Nucleation promotion for the atomic layer deposition of HfO2 onto carbon nanotubes for device applications

Abstract The potential of semiconducting single-walled carbon nanotubes (CNTs) as the next-generation semiconductor information material as a successor to silicon-based technology is promising. However, a significant challenge for CNT based electronics at device level lies in the obtainment of high-quality gate dielectrics such as HfO2 on the dangling bond-free and chemically inert surface of CNTs. To address this issue, this paper studies the atomic layer deposition (ALD) of HfO2 onto the surface of carbon-based materials, and focuses on how to promote the nucleation via two methods, i.e. low-temperature Nanofog method and high-temperature variable parameter method. Both methods are confirmed to be effective in improving the continuity and uniformity of ALD HfO2 films.

XPS study of the nitridation of hafnia on silicon

We report a comprehensive quantitative ARXPS study of 20-cycle ALD-grown HfO2 films on Si (001) substrates with and without remote plasma nitridation. The films comprised a 0.6 nm HfO2 layer atop a 1 nm hafnium-rich silicate interlayer. A detailed analysis of the Hf 4f, Si 2p, O 1s, C 1s, and N 1s core level spectra is presented for samples with and without nitridation. The fitting of the Hf 4f spectra reveals four contributing components associated with silicate, hafnia, and their respective nitrided species. The Hf 4f, O 1s, and C 1s spectra exhibit a negative binding energy shift of about 0.2 eV upon nitridation induced by a Si2+-N dipole formed at the interface. Quantitative ARXPS analysis, employing the MultiLayer Model, enabled the determination of the thicknesses, and composition together with their associated uncertainties. The analysis showed that the composition of the nitrided hafnia is HfO1.66N0.33. There is a higher degree of nitridation in the silicate interlayer compared to the overlying hafnia layer. The degree of nitridation is found to increase with plasma power; N2+ is identified as the primary plasma species responsible for the nitridation of hafnia.

Growth mechanism of HfO2 film on H-terminated Si (100) surface from atomic layer deposition process using tetrakis(ethylmethylamino)hafnium

The deposition of HfO2 on H-terminated Si (100) surfaces presents both interest and challenges due to the complexity of the surface reaction mechanisms. Previous experimental studies reported conflicting results regarding the formation of interfacial layers and the presence of incubation periods during the initial cycles of the HfO2 atomic layer deposition (ALD) process. In this study, we provide a detailed analysis of the surface reaction mechanism of HfO2 ALD on H-terminated Si (100) surfaces using tetrakis(ethylmethylamino)hafnium (TEMAH) and water. Our findings demonstrate that TEMAH chemisorbs onto the H-terminated Si (100) surface through the formation of two N-Si interfacial bonds. Additionally, the remaining ligands of the adsorbed Hf precursor are efficiently removed through hydroxylation during the water pulse, with low energy barriers of +25.9 to +30.0 kJ/mol. Furthermore, the N-Si interfacial bonds can be broken, allowing for the formation of Hf-O-Si interfacial bonds during either the water pulse or post-deposition processes. This work provides valuable insights into the surface reactions of HfO2 ALD and explains the varied interfacial bonding and incubation periods observed in prior studies.

Revealing the Properties of Electrically Driven Optical Antennas via Conductive Atomic Force Microscope.

Light emission from ultracompact electrically driven optical antennas (EDOAs) has garnered significant attention due to its terahertz modulation bandwidth. Typically, the EDOAs are fixed and nonadjustable once fabricated, thus hindering the attempts to investigate the influence of structural geometry on light emission properties. Here, we propose and demonstrate that the EDOAs can be constructed by carefully manipulating the gold-coated tips of atomic force microscopy operated in conductive mode into contact with the optical antennas covered by insulating film, where the position of the tunnel junction on the antenna surface can be controlled with high accuracy and flexibility. Taking gold nanorod antennas covered by HfO2 film as an example, we found that the highest light generation efficiency is obtained when the tunnel junction is located at the shoulder edge of the nanorod antenna, where the bonding dipolar surface plasmon mode in the junction is spectrally and spatially coupled with the longitudinal radiation mode of the EDOAs. Besides, position variation of the tunnel junction on the nanorod surface also strongly influences the far-field radiation angular distribution and emission spectrum. Numerical simulations are in good agreement with the experimental results. Our findings offer fundamental insights into the influence of structural parameters on the light emission performance of EDOAs, thus leading to better design of EDOAs with high light generation efficiency and powerful functionality.

Distinguishing the Rhombohedral Phase from Orthorhombic Phases in Epitaxial Doped HfO2 Ferroelectric Films.

Epitaxial strain plays an important role in the stabilization of ferroelectricity in doped hafnia thin films, which are emerging candidates for Si-compatible nanoscale devices. Here, we report on epitaxial ferroelectric thin films of doped HfO2 deposited on La0.7Sr0.3MnO3-buffered SrTiO3 substrates, La0.7Sr0.3MnO3 SrTiO3-buffered Si (100) wafers, and trigonal Al2O3 substrates. The investigated films appear to consist of four domains in a rhombohedral phase for films deposited on La0.7Sr0.3MnO3-buffered SrTiO3 substrates and two domains for those deposited on sapphire. These findings are supported by extensive transmission electron microscopy characterization of the investigated films. The doped hafnia films show ferroelectric behavior with a remanent polarization up to 25 μC/cm2 and they do not require wake-up cycling to reach the polarization, unlike the reported polycrystalline orthorhombic ferroelectric hafnia films.

Large ferroelectric polarization and high dielectric constant in HfO2-based thin films via Hf0.5Zr0.5O2/ZrO2 nanobilayer engineering

HfO2-based ferroelectric films have been extensively explored and utilized in the field of non-volatile memory and electrical programmability. However, the trade-off between ferroelectric polarization and dielectric constant in HfO2 has limited the overall performance improvement of devices in practical applications. Herein, a novel approach is proposed for the Hf0.5Zr0.5O2/ZrO2 (HZO/ZrO2) nanobilayer engineering, which can effectively regulate the phase structure evolution of HfO2 films to construct a suitable morphotropic phase boundary (MPB). The findings highlight that the top ZrO2 layer can regularly promote the formation of either the ferroelectric o-phase or the antiferroelectric t-phase. An ideal MPB is successfully established in HZO/ZrO2 (6/9 nm) nanobilayer film by carefully optimizing the HZO/ZrO2 thickness ratio, which presents a high dielectric constant of 52.7 and a large 2Pr value of up to 72.3 μC/cm2 without any wake-up operation. Moreover, the HZO/ZrO2 nanobilayer thin films demonstrate faster polarization switching speed (1.09 μs) and better fatigue performance (109 cycles) compared to the conventional HZO solid solution films. The relationship between ferroelectric and dielectric properties can be harmoniously balanced through the designation. The results indicate that the HZO/ZrO2 nanobilayer engineering strategy is quite potential to pave the way for the development of next-generation memory technologies with superior performance and reliability.

Contaminant-free layer-by-layer annealing to improve the properties of HfO2/SiO2 optical films for the fs laser

The low laser-induced damage threshold (LIDT) of HfO2/SiO2 films is an important factor in limiting the further development of high repetition rate femtosecond (fs) laser systems. Conventional whole-layer annealing can effectively improve the properties of SiO2 films, but it is difficult to improve the properties of HfO2 films located in the intermediate layer and is also prone to introduce contaminants. In this study, an innovative magnetron sputtering-vacuum tube furnace combined system was presented to deposit and anneal the HfO2/SiO2 films without contaminant. The layer-by-layer annealing optimizes the stoichiometric ratio, stress, and surface morphology of HfO2/SiO2 films. The fs laser damage test at a high repetition rate indicated that the LIDT of the layer-by-layer annealed films reached 1.15 J/cm2 and 1.99 J/cm2 at 515 nm and 1030 nm, which were about 28% and 25% higher than those of the un-annealed films, respectively. This work provides a way to effectively avoid the contaminant introduction during annealing and improve the damage threshold of bilayer films by layer-by-layer annealing in the magnetron sputtering-vacuum tube furnace combined system.

The Evaluation of Interface Quality in HfO2 Films Probed by Time-Dependent Second-Harmonic Generation.

Time-dependent second-harmonic generation (TD-SHG) is an emerging sensitive and fast method to qualitatively evaluate the interface quality of the oxide/Si heterostructures, which is closely related to the interfacial electric field. Here, the TD-SHG is used to explore the interface quality of atomic layer deposited HfO2 films on Si substrates. The critical SHG parameters, such as the initial SHG signal and characteristic time constant, are compared with the fixed charge density (Qox) and the interface state density (Dit) extracted from the conventional electrical characterization method. It reveals that the initial SHG signal linearly decreases with the increase in Qox, while Dit is linearly correlated to the characteristic time constant. It verifies that the TD-SHG is a sensitive and fast method, as well as simple and noncontact, for evaluating the interface quality of oxide/Si heterostructures, which may facilitate the in-line semiconductor test.

Structural and Ferroelectric Transition in Few-Layer HfO2 Films by First Principles Calculations

Abstract The discovery of ferroelectricity in HfO2-based materials with high dielectric constant has inspired tremendous research interest for next-generation electronic devices. Importantly, films structure and strain are key factors in exploration of ferroelectricity in fluorite-type oxide HfO2 films. Here we investigate the structures and strain-induced ferroelectric transition in different phases of few-layer HfO2 films (layer number N = 1–5). It is found that HfO2 films for all phases are more stable with increasing films thickness. Among them, the Pmn21 (110)-oriented film is most stable, and the films of N = 4, 5 occur with a P21 ferroelectric transition under tensile strain, resulting in polarization about 11.8 μC/cm2 along in-plane a-axis. The ferroelectric transition is caused by the strain, which induces the displacement of Hf and O atoms on the surface to non-centrosymmetric positions away from the original paraelectric positions, accompanied by the change of surface Hf–O bond lengths. More importantly, three new stable HfO2 2D structures are discovered, together with analyses of computed electronic structures, mechanical, and dielectric properties. This work provides guidance for theoretical and experimental study of the new structures and strain-tuned ferroelectricity in freestanding HfO2 films.